A light can be faster but harder to drive. We use weight at the front to stop the car from lifting the front wheels and some at the back to aid rear traction - its like a tuning aid.
Some cars have not been designed around LiPo and the power they deliver/ lighter weight. I personally use additonal brass weights under the battery and servo to try and balance the B4's handling. I had to play around with this when we switched from NiMH to LiPo and I was not the only one who struggled with the change over. Its a case of personal preference as some of the faster drivers can run lighter than others as they have the skills to match.
I agree with what Tony says that for me, the additional weight makes the car easier to drive and therefore I am less likely to crash...
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